A group III nitride based compound semiconductor (a gallium nitride (GaN), an indium gallium nitride (InGaN), or a gallium aluminum nitride (GaAlN), etc.: which will be also simply referred to as a nitride semiconductor hereinafter) has recently started playing an important role as a material for a blue or ultraviolet light emitting diode (LED) or a laser diode (LD). Further, since a nitride semiconductor is superior in heat resistance or environment resistance or in high-frequency characteristics besides optical devices, electronic devices exploiting such characteristics have been positively developed.
However, bulk crystal growth of a nitride semiconductor is difficult, and a GaN self-supporting substrate is just utilized for, e.g., a laser diode application having no problem in cost in a limited way. A substrate for GaN growth which is currently extensively in practical use is a sapphire (Al2O3) substrate, and a method for epitaxial growth of GaN on a single-crystal sapphire substrate based on, e.g., a metalorganic vapor phase epitaxy method (an MOVPE method) is generally used.
In this case, since the sapphire substrate has a lattice constant different from that of GaN, a single-crystal film cannot be grown when GaN is directly epitaxially grown on the sapphire substrate. Therefore, a method for temporarily growing a buffer layer of AlN or GaN on the sapphire substrate at a low temperature, relaxing lattice distortion by using the buffer layer grown at a low temperature, and then growing GaN on this buffer layer is suggested (Japanese Patent Application Laid-open No. S61-188983)
However, even in growth of GaN using this buffer layer grown at a low temperature, the substrate after the epitaxial growth warps due to a difference in thermal expansion coefficient between the sapphire substrate and GaN, resulting in a problem of cracks or breakage.
Furthermore, warpage of the substrate after the epitaxial growth makes an exposure state of a fine pattern in photolithography uneven, thereby leading to a serious problem.
Moreover, in an illumination blue or ultraviolet LED that is demanded to be put to practical use in future, because the LED must emit light at a high luminance with a high current density, a low-cost GaN self-supporting substrate having a low dislocation density of a GaN light emitting layer and an excellent thermal conductivity with respect to the substrate is desired in terms of a light emission efficiency and a life duration.
Although a growth method for a GaN self-supporting substrate superior in crystallinity and productivity is demanded as explained above, a satisfactory countermeasure is yet to be provided.
To solve such a problem, an attempt of removing a sapphire substrate from a GaN epitaxially-grown substrate thickly grown on the sapphire substrate by a method of, e.g., etching or grinding to obtain a self-supporting substrate of GaN has been also made. When the self-supporting substrate of GaN is obtained, various problems caused due to a difference in lattice constant or a difference in thermal expansion coefficient in the epitaxial growth for forming a light emitting layer can be solved.
However, there still remains a problem that an inner strain of the GaN epitaxial layer due to a difference in thermal expansion coefficient between sapphire and GaN is locally relieved when the sapphire substrate is removed, and warp of the GaN substrate is thereby increased, thus breaking the substrate. A practical application of a method for thickly growing GaN on a sapphire substrate based on an HVPE method (Hydride Vapor Phase Epitaxy) and then applying a laser pulse to delaminate a GaN layer alone has been attempted (e.g., Jpn. J. Appl. Phys. Vol. 38 (1999) pt. 2, No. 3A, L217-219), but this method has a disadvantage that the substrate is apt to be cracked during a delamination process, thereby leading to a problem that a complicated processing is required to obtain a large GaN substrate with excellent reproducibility.
Additionally, a method for using a single crystal such as LiAlO2 or ZnO whose lattice constant is close to that of GaN as a substrate in place of a sapphire substrate and growing GaN has been suggested. When such a substrate is used, delamination of the substrate becomes relatively easy, but heteroepitaxial growth is surely provided. Therefore, a buffer layer is required, and a practical application of a GaN substrate having excellent crystallinity still has a problem because of a difference in growth temperature or melting point of the substrate.
Further, a method for forming a mask of, e.g., Si3N4 having a window in a GaAs substrate, forming a low-temperature buffer layer, then epitaxially growing in a lateral direction based on the HVPE method to form an epitaxial layer having a low dislocation density, and removing the GaAs substrate by, e.g., etching to obtain a GaN self-supporting substrate has been carried out (Japanese Patent Application Laid-open No. 2000-12900 and Japanese Patent Application Laid-open No. 2000-22212). However, this method requires, e.g., a process of forming an Si3N4 mask having a window or a process of forming a low-temperature buffer layer. Furthermore, there is also a problem that the GaN self-supporting substrate greatly warps.
Moreover, since GaN epitaxial growth can be carried out at a relatively high speed in the HVPE method, an attempt of slicing a single-crystal ingot i.e., a boule formed by epitaxially growing an ultrathick film having a thickness of approximately 1 cm to 10 cm or above on a GaN self-supporting substrate based on such characteristics to obtain many substrates (sliced substrates) and polishing slice surfaces of the sliced substrates to acquire many GaN self-supporting substrates (which will be referred to as a boule method hereinafter) is also carried out (see, e.g., Japanese Patent Application Laid-open No. 2000-12900 and Japanese Patent Application Laid-open No. 2000-22212). However, according to this method, stably obtaining the GaN self-supporting substrate having a high crystal quality is difficult.